Tower defense is a minigame for the spring rts engine where you have a pathway though which units will travel. The goal is to kill "creeps" with stationary turrets prior to them completing the course.
To build turrets you will spend funds. Each turret has a certain cost and the currency for the cost is acquired by killing the "creeps." Also each turret can be upgraded at a cost for high effectiveness.
Creeps will become stronger and vary in their attempts to exit the level quickly. Exterminate them all!